1) Field of the Invention
The present invention relates to a procedure of accurately weighing the metal content in a vessel in a casting plant provided with equipment for heating/stirring the melt by inductive methods.
2) Description of Related Art
The accuracy of weighing in such applications until now has been unsatisfactory due to influence from magnetic forces arising from the induction current as well as variations of metal bath level and lining profile of the vessel, i.e., wear. These variations in turn influence the size of the magnetic forces.
DE-A1-2513923 relates to automatic controlling equipment for the preparation of metallic crystals where continuous weighing of the inductively heated vessel containing the liquid metal is performed. Compensation for the disturbance on the weighing result caused by the forces of the magnetic field has been introduced via a correction signal generator in which the influence of the disturbing force is calculated as a constant times the coil current. The correction signal generated in this way is based on a simplified theory about the origination of the disturbing force and gives no true imitation of the disturbing force during the whole casting procedure and therefore does not permit accurate enough control of the metal flow in applications where this is required. Simultaneous weighing of both vessel with metal content and heating equipment characterizing the present invention is not taught in DE-A1-2513923.
Methods of making spray-cast billets or metal powder require accurate control of the temperature of the liquid metal and of the metal flow rate. Common to all methods used is the forming of a spray of atomized metal by blowing gas jets at high speed against a liquid metal stream which disintegrates into droplets. Properties such as powder size, billet structure, grain size, porous layer thickness, etc., of gas atomized metal powder or spray-cast billets are primarily determined by the temperature and mass flow of the liquid metal during the formation of the atomized metal flow.
Methods for controlling or keeping constant the temperature or flow rate of the atomizing gas represent well-known techniques.
Controlling or keeping constant the temperature of the liquid metal during atomizing can be performed by using an inductively heated and sealed cladle. Control of the temperature of the liquid metal in such a ladle is obtained by measuring the temperature of the melt and varying the power to the induction coil. This is a known technique which has been described in SE-A-8006244-1.
The known methods and techniques for controlling the liquid metal flow through the opening of the outlet (nozzle) of an inductively heated ladle do not permit sufficient precision, however. Accurate control of the metal flow rate at the tapping of metal from a ladle is a necessity. The reason for this is that the metal has to be deposited with great precision in order to give correct and near net (final) dimension of the manufactured object. Disturbances such as clogging of the nozzle opening, affecting the deposition procedure have to be compensated for. The methods according to known technique being used so far are based upon measuring the level of the metal surface in the ladle and do not consider the lining wear of the ladle, the flow rate of the liquid inside the ladle or the wear of the nozzle opening. These methods of controlling the metal flow are based upon the theoretical relation between static pressure and flow rate derived from Bernoulli's equation. Because of factors such as temperature, viscosity of the melt, type of ceramic material, and chemical reactions between the melt and the ceramic lining, the real flow rate deviates from that which is theoretically calculated.
Another method of controlling the metal flow rate is to weigh the ladle including its metal content and calculate the total weight decrease per time-unit. The weighing equipment has to meet high demands with respect to resolution and accuracy, since the metal flow rate (weight decrease per time-unit) is so small relative to the weight of the total load and the requirement of keeping the flow constant is so great. A current requirement in the industry is that it has to be able to detect weight changes of 0.5 kg, which should be compared with the total weight of ladle and melt of about 4 metric tons of which the melt represents about 1 metric ton. The weighing operation is exposed to disturbances from the magnetic force emanating from the current in the induction coil for stirring and heating, which directly influences the weighing result. This disturbing force varies in an interval between 0 kg (at cut-off power supply) and up to about 40 kg depending upon the conditions prevailing in the process as described below.
In this prior manufacturing process, compensation was initially introduced based upon an empirical model to correct the weight signal. This model calculates the disturbing force at the actual steel bath level and coil current. Steel tubes manufactured by means of the model showed, however, that the correction factor was influenced also by the profile of the ladle lining and that the level of the steel bath could not be defined as function of the coil frequency (being used as measure of the bath level) with sufficient accuracy. The model was modified in order to consider the inner diameter of the ladle too, but was abandoned because of its complexity and the unsatisfactory results obtained.